Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Patent
1990-09-26
1993-07-27
Lateef, Marvin M.
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
338 21, H01C 710
Patent
active
052313719
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a circuit protection device including a positive temperature coefficient resistor (hereinafter called a polymer PTC element) wherein carbon black is dispersed in crystalline resin.
BACKGROUND OF THE INVENTION
In a series circuit composed of a load, a power source, and a polymer PTC element, when an overcurrent flows such as by short-circuiting the temperature of the polymer PTC element rises due to Joule heat generation and in dependence upon the temperature rise the resistance of the element increases. As a result, the overcurrent is limited and the circuit is protected.
The polymer PTC element, as disclosed in Japanese Patent Application Laid-Open publication No. 216401/1986, is a resistor wherein the carbon black is dispersed in crystalline resin and shows a positive temperature coefficient. Further, it has been known that such a polymer PTC element has an overcurrent protecting function.
FIG. 2 is a circuit diagram illustrating a circuit protection device including a polymer PTC element. In the circuit of the same drawing, when an overcurrent flows due to short-circuiting, and assuming that the resistance of the load is R and the current flowing in the load is I, the amount of heat generation I.sup.2 R in the polymer PTC element exceeds the amount of heat radiation, and the temperature of the element rises. Since the polymer PTC element has a resistance-temperature characteristic as shown in FIG. 3, the resistance thereof increases in dependence upon the temperature rise, the current is limited and the circuit in the form of a road is protected.
FIG. 4 is a diagram illustrating a current-voltage characteristic of the polymer PTC element. In a region where an applied voltage to the polymer PTC element is low, the temperature of the element is low and Ohm's low is maintained. In the circuit of FIG. 2, when there is no abnormality in the circuit, the polymer PTC element is held in any optional regions. Such regions correspond to steady state, and the current at this moment is called a steady current. When the applied voltage to the polymer PTC element further increases, the resistance of the polymer PTC element will suddenly increase while the temperature thereof is kept substantially constant. After the current exceeds the largest working current which can flow in the non-operable range of the polymer PTC element and reaches to a largest value, the current attenuates in response to the voltage rise. This largest current is called a maximum current. When an overcurrent flows through the circuit, the amount of heat generation of the polymer PTC element exceeds the amount of heat radiation, and the temperature of the polymer PTC rises. The amount of heat radiation is proportional to the difference between the element temperature and the ambient temperature. Since the resistance suddenly increases and the current decreases, finally, the amount of heat generation and that of heat radiation become equal (equilibrium condition). The current at this moment is called an equilibrium current. The relationship between the amount of heat generation and that of heat radiation is expressed by the following equation wherein C is the heat radiation coefficient, T is the element temperature and T.sub.a is ambient temperature:
In the region where the resistance of the polymer PTC element rises, the temperature of the element is maintained substantially constant and the current I decreases until the amount of heat generation coincides with that of heat radiation.
For such use, a PTC using a barium titanate system is employed widely. The resistance of the PTC of barium titanate is high so that the size of the element becomes large to increase the heat radiation coefficient. For example, with a polymer PTC element employed for the overcurrent protection use for a small size motor, it displays a current-voltage characteristic A as shown in FIG. 1. When a power source voltage of 10 V is used, the equilibrium current becomes 0.5 A which is larger than the steady current
REFERENCES:
patent: 4481498 (1984-11-01), McTavish et al.
Lateef Marvin M.
TDK Corporation
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